Hi there... my doc said that the reason they do the u/s at 20 wks is because everything is fully formed and the baby is still small enough and has plenty of room to move, making the "parts" alot easier to see. I too, just had an u/s at 28 weeks to check the placenta (today) and the doc couldn't see my baby's kidneys very well. However, at 20 weeks, the bladder and kidneys looked great, so he wasn't worried at all. I think the bigger and more crowded they get, it's a little less easier to view everything. Especially after they ball up and assume the fetal position. So, I'd say, if everything was fine at 20 weeks, everything is fine now. I'm sure they would have found a spinal problem if there was one at 20 weeks, just like my baby's kidney's wouldn't dissapear! Try not to worry.